B. D. Cahan

Structural effects in electrocatalysis: oxygen reduction on platinum low index single-crystal surfaces in perchloric acid solutions

Oxygen reduction has been studied in 0.1 M HClO4 on the platinum low index surfaces, employing a hanging meniscus rotating-disk technique. A special cleaning procedure has been developed to remove impurities from the surface prior to the oxygen reduction measurements. Oxygen reduction was found to be sensitive to the crystallographic orientation of the platinum electrode surface. The activity for oxygen reduction discerned from the half-wave potential decreases in the sequence (110) & >; (111) & >; (100). The reaction proceeds on all well-ordered low index planes with exchange of four electrons per O2 molecule. Tafel slopes of −120 mV per decade at high current densities and −60 mV / per decade at low current densities were found for all three planes at room temperature. Oxygen-containing species chemisorbed on platinum appear to be the cause of the change in the Tafel slope.

Optical and Electrochemical Studies of Underpotential Deposition of Lead on Gold Evaporated and Single‐Crystal Electrodes

Abstract : Linear sweep voltammetry and reflectance spectroscopy have been used to examine the under potential deposition of lead on gold in Pb(2+) containing HC104 solutions. The voltammetry curves and reflectance change data provide evidence that the lead is first deposited as ions although their effective ionic charge is probably reduced substantially from +2 through their strong interaction with the band structure of the gold substrate. At more cathodic potentials, still well below the reversible potential of bulk lead, a sharp transition is observed over a 5 to 10 mV range. This transition appears to involve a two dimensional phase transition leading to a metallic-like lead layer. Adsorption isotherms have been evaluated from the reflectance changes. A.c. electromodulation techniques and complex plane analysis have been used to examine the kinetics of the lead adsorption-desorption and to evaluate the apparent exchange current density for the process. (Modified author abstract)

The Nature of the Passive Film on Iron II . A‐C Impedance Studies

The passive film on iron in borate buffer has been studied with wide frequency range impedance techniques. Two well‐defined potential domains are observable, i.e., above and below 1.0V vs. the reversible hydrogen electrode in the same solution (RHE). A discussion of the interpretation of electrochemical impedances and of interfering artifacts is presented. An analysis of the impedance curves raises serious questions about the interpretation of the data in terms of a semiconductor model.

LEED-AES-Thin Layer Electrochemical Studies of Hydrogen Adsorption on Platinum Single Crystals.

Abstract The understanding of the relation of electrocatalysis to electrode surface structure requires measurements on electrode surface free of impurities and of well defined morphology. A system has been developed which allows preparation and characterization of such surfaces in an ultrahigh vacuum using LEED and AES, their electrochemical examination within a thin-layer cell, and subsequent re-examination of the surface morphology and purity. The electrochemical adsorption-desorption of hydrogen has been studied on the three low-index planes of Pt in dilute HF. When the potential range is restricted to 0.05–0.5 V vs. RHE (1 atm), post-electrochemical LEED analysis shows that the Pt(111) and (110)-(1×2) surfaces retain their original structure while the Pt (100)-(5×20) loses its (5×20) overlayer.

The hanging meniscus rotating disk (HMRD)

A new method for mounting cylindrical samples for use as rotating disk electrodes which eliminates many of the problems associated with more conventional techniques has been developed

Electrode potential scanning ellipsometric spectroscopy: Study of the formation of the anodic oxide film on noble metals

Abstract The formation of the anodic oxide film on Pt has been investigated at wavelengths in the visible using voltage scanning ellipsometry. Changes in light intensity in an ellipsometric experiment have been used to determine the changes in Δ0, ψ0, and reflectivity of the surface. From these three parameters, the effective complex refractive index and thickness of the surface film have been calculated, using a three-layer model. The oxide film on Pt has been found to have different optical properties above and below 1.1 V versus NHE. The apparent film thickness increases with constant optical properties between 1.1 and 1.6 V, indicating that the oxide may form in patches on the surface. The optical constants of this film show small dependence on wavelength in the visible. The anodic oxide film on Au has also been studied using this technique. The presence of the optical absorption edge of Au in the visible region of the spectrum causes the Au surface to be very sensitive to interactions with the surface oxide. This results in a failure of the simple three-layer model at wavelengths near the absorption edge of Au.

Modulation ellipsometry and its application to the study of the electrode-electrolyte interface

Abstract A methodology has been developed for following rapidly small changes of the ellipsometric parameters Δ 0 and ψ 0 and of the reflectivity necessary for the unique determination of n = n − i k and thickness of a surface film. Intensity changes at given settings of the polarizer and analyzer are quantitatively related to these parameters. Examples are given of the application of this technique to the electrochemical interface of Pt and Au electrodes.

Implications of three parameter solutions to the three-layer model

The use of relative reflectivity changes as an independent third ellipsometric parameter provide insight into shortcomings of the three layer model in thin film systems. An analysis of the successes and failures of this approach in the study of thin films produced electrochemically is presented. An algorithm for the rapid computer evaluation of two (Δ and ψ) and three (Δ, ψ, and δR/R) parameter value for n, k and t of an absorbing film is discussed.

An Ellipsometric Investigation of the Underpotential Deposition of Lead on Gold

Abstract : Potential scanning ellipsometry has been used to examine the wavelength and potential dependence of the optical constants of the lead layer formed on Au by underpotential deposition from Pb(2+) solution. The two main current peaks in the voltammetry curves of the system can be identified with two different states of the surface film. Neither state has optical constants resembling those of bulk Pb. Both films have an ellipsometrically determined thickness of 1.5 A. The large wavelength dependence of the optical constants of the films indicates a strong interaction of the lead adlayer with the surface layer of the Au substrate. The ellipsometric data are consistent with a model in which the layer formed on the surface at less cathodic potentials is primarily ionic and at cathodic potentials undergoes a transition to a metal-like lead adlayer.

The effects of trace anions on the voltammetry of single crystal gold surfaces

Abstract The development of a new technique for mounting single crystals as a Hanging Meniscus Rotating Disk (HMRD) electrode has facilitated the study of the effect of traces of anionic impurities on the voltammetry of gold single crystals. It is shown that the cause for most of the differences between voltammetry curves reported in the literature for the same crystal face is the presence in the solution of low levels (≈ 10−7 M) of Cl− and SO42− ions which were present in the original reagent. Variation of the rotation rate from 0 to 10,000 r.p.m. permitted a change of the effective concentrations of these impurities by a factor of ≈100. The resultant curves are compared to those produced by deliberate addition of these anions. The results obtained indicate that the previously reported specific adsorption of ClO4− and F− on Au may well have been due to the presence of the same contaminants (Cl− and SO42−). The capacitance minimum values reported in the literature as pzcs appear to coincide with the potential at which SO42− (or HSO4−) desorption finishes and Cl− begins. The reported values for pzc may be erroneous. The validity of the “fingerprint” concept is discussed, as is the use of modifications (changes) of the voltammetric features produced by particular pre-polarisation regimes as electrochemical evidence of “reconstruction”.